Came back from my trip a few weeks ago. Had a great time and I highly recommend everyone go at least once to Hawaii (Big Island). It's certainly got a different feel to Oahu. Having now been to each of the islands of Hawaii, it seems each island has it's own "personality". They're all certainly unique in their own way and you won't get the "meh, I've seen this before" feeling when visiting.
I ended up splitting my trip into 4 days Kona and 3 days in Hilo so I could cut out the back and forth driving. Checked out the following places:
*Hapuna Beach & Anaeho'omalu Beach
*Pololu Valley Lookout & Waipi'o Valley Lookout
*South Point, Papakōlea (Green Sand Beach) & Punalu'u (Black Sand Beach)
*Hawaii Tropical Botanical Gardens
*A'kaka Falls, Rainbow Falls, Umauma Falls
*Volcanoes National Park
*Mauna Kea
I also did the Manta Ray snorkeling but unfortunately, on the day I went, there were no manta rays so I struck out on that, and couldn't reschedule. I'll have to try that next time.
The highlight of the trip was obviously the volcanoes. I took the boat tour out to the point where the lava flows into the Pacific Ocean - I definitely recommend it if you do want to check it out. It's very bumpy and you do get wet so it's not for everyone but the view is spectacular and it gives a different perspective (and a lot less strenuous) than walking out there (and even the helicopter tours)
I also took a tour up to the peak of Mauna Kea. It was a "sunset tour", so you're going up there in the late afternoon and watching the sun set. Again, another trip I would recommend if you have the opportunity. Again, another bumpy ride, and the air gets very thin up there. (where even walking up a hill gets you pretty winded) However, the views are spectacular.
Papakōlea (Green Sand Beach) now has locals who will offer you rides out to the beach for $25 round trip. I ended up taking them up on the offer for a ride and I have to say, it's worth the money. It's a long, hot, windy walk out to the beach.
I didn't get to snorkel at Kealakekua Bay as I ran out of time. I ended up snorkeling at Kahaluʻu Beach on my final day there and managed to see a sea turtle and a few fish.
Overall, I think as CivicBlues mentioned, a week is probably good to explore Big Island. I ended up with a bit of spare time on the last day at each location so I could actually relax a bit and slow down.
Lava flowing into the ocean. Did the 4:30am tour - probably the best time to do it since you can get a better view of the lava and catch the sunrise on the way back.
Mauna Kea Sunset. Only some tour companies and true 4x4 vehicles can drive up to the peak. Pretty awesome sunset with Haleakala Crater on Maui in the background.
Papakōlea (Green Sand) Beach.
Punalu'u (Black Sand) Beach
Sea Turtle at Kahalu'u Beach